The two cases are absolutely identical except for the bezels and PSUs. The D8000 is probably a better deal if you're going to use your own PSU or don't like the door or color of the SLK3700. My preference is for the 3700 though. I like the door because it keeps things a little quieter and I took out the Antec PSU and sold it for $35. I ended up with the 3700 for $40 after I sold the PSU.
I don't know about the D8000 case fans, sorry. Here's what the Coolcases website says about the fans that are included with the D8000:
The factory fans supplied are 'Global' brand, 120x25 sleeve bearing fans. They have a 3- pin/mbd female connector, a 10" lead on the remote thermistor, and are true variable-speed fans.
Maximum speed (12v.) specs are 64 cfm, 34 dBA.
So they may be fairly quiet at their default (slow) speed.